The overall objective of the research is to define the association between subclinical hypothyroidism and prolactin activity and human cancer. Attention is being paid to the role which these factors may have both in the etiology of breast cancer and the prognosis. Plasma thyroid-stimulating hormone (TSH), thyroxin and prolactin levels are being assayed, and testing for thyroid antibodies performed, and in two clinical situations: (1) in apparently healthy women who are taking part in a breast cancer screening program and (2) in patients awaiting mastectomy or about to be entered into a clinical trial of adjuvant chemotherapy. Thyrotropin-releasing hormone stimulation tests, with assays of plasma TSH and prolactin, are also being carried out in candidates for adjuvant chemotherapy; patients with demonstrated regional lymph node involvement at mastectomy. The pituitary - thyroid studies will be complemented by appropriate follow-up procedures, and the results correlated with on-going studies of plasma and urinary steroid hormones in these patients.